Only the finish (top) coat of plaster is hard as nails; the brown (middle) coat is softer, and although the scratch (deepest) coat can be slightly harder than the brown coat, it’s still not rock-hard like the finish coat.
Your typical USA pre-1920 plaster job contains lime in all three coats, sharp sand in the brown and scratch coats, and animal fibers (typically horse hair or pig bristles but sometimes cattle hair) in the scratch coat. Occasionally there is stone dust in the finish coat. When cut or smashed, all these things will create dust that you should not breathe - in particular you want to avoid inhaling powdered pig bristles - because although they are not conventionally poisonous, they will nonetheless do permanent harm to your lungs. A cheap dust mask is sufficient.
To examine a secret room or priest’s hole, use a 1.5" or larger hole saw to create an opening you can push a camera and light through. Understand that if you hit a stud you’ll probably destroy the hole saw in one use, because you’re quite likely to hit multiple nails in the lath. Even if you don’t hit any nails you’ll still wreck a cheap hole saw because of the sand in the brown coat. Do not just drill until you stop feeling any resistance, cut through each layer separately, so you don’t snarl free-hanging wires, cut into a pipe, or sever a stud. Masking tape on the finish coat will reduce the chance of a running crack, and is very much worth doing. Wetting the brown and scratch coats with a spritzer bottle will reduce dust, ditto.
Wooden lath is sawn or riven or split. If it’s not sawn, you have a very historic wall and you should under no circumstances destroy it without first getting significant amounts of input from local historians.
Once you can look inside with a small USB OTG camera & LED light attached to your cell phone, you’ll find it’s either an equipment chase (for wires and plumbing and gas lines) or a secret lair of Nyarlathotep. In the latter case, you should be able to see at least one sealed doorway. If there is only one, do not open this door or remove the Elder Sigil. However, if there is a doorway that leads into the mortal plane, let me know and I’ll tell you how to reopen it.
